The following invention relates to a variable impedance driver circuit for providing test signals from automatic test equipment in order to stimulate a device under test.
In driver circuits of the type used with automatic test equipment, it is often advantageous to use CMOS integrated chip technology to build the drivers needed to provide the test signal pulses used to stimulate a device under test because of the relative inexpensiveness and compact size of CMOS IC's. The device under test is usually connected to the driver output by a fixed impedance transmission line. Thus, in order to maintain the desired shape of the test pulses, it is necessary to match the output impedance of the driver circuit to the impedance of the transmission line.
One problem with such drivers, however, is that due to performance variations in the CMOS chips, the impedances of such drivers may vary considerably. It is not uncommon for the impedances of similar CMOS devices made by the same manufacturing process to vary as much as 100%. What is needed, therefore, in a test instrument using CMOS transmission gates as output drivers, is a way to match the impedance of each driver to the transmission line which is to be connected to the device under test.